1. Field of the Invention
The present invention relates to a valve seat, which is a structural member of an internal combustion engine such as a diesel engine or a gasoline engine, and relates particularly to an Fe-based sintered alloy valve seat (hereafter simply described as a valve seat) that exhibits excellent wear resistance under conditions of high surface pressure application.
2. Description of the Related Art
The cylinder heads of internal combustion engines such as diesel engines or gasoline engines are provided with valve seats for the exhaust valve and the intake valve.
Conventionally, the valve seats utilize an Fe-based sintered alloy which has an overall composition comprising, in terms of weight percentage (hereafter all % values relating to compositions refer to % by weight),
C: 0.7 to 1.4%,
Si: 0.2 to 0.9%,
Co: 15.1 to 26%,
Mo: 6.1 to 11%,
Cr: 2.6 to 4.7%,
Ni: 0.5 to 1.2%,
Nb: 0.2 to 0.7%,
and a balance of Fe and inevitable impurities, wherein
a substrate formed from an Fe-based sintered alloy, comprising a composition in which a hard dispersion phase formed from Coxe2x80x94Moxe2x80x94Cr alloy is distributed in an Fe-based alloy matrix, and
having a porosity of 5 to 15%,
is infiltrated with copper or copper alloy to form the valve seat (for example, refer to the patent reference 1).
Furthermore, it is known that the valve seat described above can be produced using, as the raw material powder for forming the matrix, an Fe-based alloy powder with an average particle size of 75 to 107 xcexcm, and comprising:
C: 0.8 to 2.1%,
Ni: 0.6 to 1.7%,
Cr: 1.2 to 3.6%,
Nb: 0.3 to 0.9%,
Co: 4.3 to 13%,
Mo: 1.4 to 4.2%,
and a balance of Fe and inevitable impurities, and using, as the raw material powder for forming the hard dispersion phase, a Co-based alloy powder with an average particle size of 68 to 102 xcexcm, and comprising:
Mo: 20 to 35%,
Cr: 5 to 10%,
Si: 1 to 4%,
and a balance of Co and inevitable impurities,
by conducting solid phase sintering, in an ammonia cracked gas atmosphere, of a pressed compact formed from a mixed powder generated by mixing the Co-based alloy powder into the Fe-based alloy powder in sufficient quantity to account for 25 to 35% by weight of the combined weight with the Fe-based alloy powder, thereby forming an Fe-based sintered alloy substrate,
and then infiltrating this Fe-based sintered alloy substrate with copper or a copper alloy (refer to patent reference 1).
Japanese Unexamined Patent Application, First Publication No. Hei 11-209855 A
On the other hand, the increase in the size and output of internal combustion engines in recent years has been remarkable, and accompanying these trends, the spring constant of the valve springs have tended to increase with the aim of preventing gas leakage of the combustion gases. As a result, the seat load applied to the valve contact surface of the valve seat increases even further, meaning operation of the valve seat under conditions of high surface pressure application is unavoidable, but when a conventional valve seat such as that described above, or any of a variety of other valve seats, is used under conditions of high surface pressure application, wear of the valve seat is accelerated considerably, meaning the valve seat reaches the end of its life in a comparatively short time.
Taking the above circumstances into consideration, the inventors of the present invention conducted research into developing a valve seat that exhibits excellent wear resistance, even when used under conditions of high surface pressure application, and made the following discoveries (a) to (c).
(a) The reason that the conventional valve seat described above displays inadequate wear resistance under conditions of high surface pressure application is that because the adhesion of the hard dispersion phase to the matrix is unsatisfactory, the hard dispersion phase readily separates from the matrix under conditions of high surface pressure application, causing an acceleration of the wearing process.
(b) The Fe-based sintered alloy substrate used to form the conventional valve seat described above is produced, as described above, using an Fe-based alloy powder for forming the matrix, and a Co-based alloy powder for forming the hard dispersion phase, both with an average particle size of 68 to 107 xcexcm, by conducting sintering in an ammonia cracked gas atmosphere, and as a result, in the Fe-based sintered alloy substrate generated following sintering, the matrix has essentially the same composition as that of the Fe-based alloy powder used for forming the matrix, and similarly, the hard dispersion phase has essentially the same composition as that of the Co-based alloy powder used for forming the hard dispersion phase, whereas if the sintering atmosphere is altered to a vacuum atmosphere (a reduced pressure atmosphere), and the particle sizes of the raw material powder for forming the matrix and the raw material powder for forming the hard dispersion phase are reduced to average particle sizes within a range from 20 to 50 xcexcm, and in addition, if the raw material powder for forming the matrix utilizes an Fe-based alloy powder comprising:
C: 0.5 to 1.5%,
Ni: 0.1 to 3%,
Mo: 0.5 to 3%,
Co: 3 to 8%,
Cr: 0.2 to 3%,
and a balance of Fe and inevitable impurities, and the raw material powder for forming the hard dispersion phase utilizes a Co-based alloy powder comprising:
Mo: 20 to 32%,
Cr: 5 to 10%,
Si: 0.5 to 4%,
and a balance of Co and inevitable impurities, then during sintering, the Co, Cr and Si components of the Co-based alloy powder diffuse and migrate into the matrix, and the Fe component of the Fe-based alloy powder diffuses and migrates concurrently into the gaps in the Co-based alloy powder left by the migration of the Co, Cr and Si components, thereby generating a mutual diffusion and migration phenomenon of the alloy components.
(c) The Fe-based sintered alloy substrate generated during the sintering described in (b) above, in which the alloy components have undergone mutual dispersion and migration between the matrix and the hard dispersion phase, is formed from an Fe-based sintered alloy with a porosity of 10 to 20%, and comprising, according to measurements performed using an X-ray microanalyzer (EPMA), an Fexe2x80x94Co alloy matrix comprising:
C: 0.5 to 1.5%,
Ni: 0.1 to 3%,
Mo: 0.5 to 3%,
Co: 13 to 22%,
Cr: 1 to 5%,
Si: 0.1 to 1%,
and a balance of Fe and inevitable impurities, in which is uniformly distributed a hard dispersion phase of a Moxe2x80x94Fexe2x80x94Co alloy, having a composition comprising:
Fe: 20 to 30%,
Co: 13 to 22%,
Cr: 1 to 5%,
Si: 0.3 to 3%,
and a balance of Mo and inevitable impurities, and having a 2 phase mixed system of an Fexe2x80x94Co alloy phase and a Moxe2x80x94Co alloy phase, and from these findings it is evident that as a result of this mutual diffusion and migration of large quantities of the alloy components between the matrix and the hard dispersion phase, the adhesion of the hard dispersion phase to the matrix improves markedly, and moreover, the matrix displays excellent high temperature corrosion resistance in the fuel combustion gas atmosphere, and the hard dispersion phase has superior high temperature hardness and displays excellent high temperature corrosion resistance, and consequently, the Fe-based sintered alloy substrate described above exhibits excellent wear resistance as a valve seat, even under high surface pressure application conditions, and if the substrate is infiltrated with copper or a copper alloy then the thermal conductivity and the strength of the substrate can be further improved.
The findings in (a) to (c) above summarize the results of the research conducted by the inventors.
The present invention is based on the research results described above, and provides a process for producing a valve seat comprising the steps of:
(a) using, as a raw material powder for forming a matrix, an Fe-based alloy powder comprising:
C: 0.5 to 1.5%,
Ni: 0.1 to 3%,
Mo: 0.5 to 3%,
Co: 3 to 8%,
Cr: 0.2 to 3%,
and a balance of Fe and inevitable impurities, and having an average particle size of 20 to 50 xcexcm; and using, as a raw material powder for forming a hard dispersion phase, a Co-based alloy powder comprising:
Mo: 20 to 32%,
Cr: 5 to 10%,
Si: 0.5 to 4%,
and a balance of Co and inevitable impurities, and having an average particle size of 20 to 50 xcexcm,
(b) conducting solid phase sintering, under vacuum, of a pressed compact formed from a mixed powder generated by mixing the Co-based alloy powder into the Fe-based alloy powder in sufficient quantity to account for 25 to 35% by weight of the combined weight with the Fe-based alloy powder, and causing the Co, Cr and Si components of the Co-based alloy powder to diffuse and migrate into the matrix, and the Fe component of the Fe-based alloy powder to diffuse and migrate concurrently into the hard dispersion phase, thereby markedly improving the adhesion of the hard dispersion phase to the matrix, and forming, as a result, an Fe-based sintered alloy substrate with a porosity of 10 to 20%, and comprising, according to measurements performed using an X-ray microanalyzer (EPMA), an Fexe2x80x94Co alloy matrix comprising:
C: 0.5 to 1.5%,
Ni: 0.1 to 3%,
Mo: 0.5 to 3%,
Co: 13 to 22%,
Cr: 1 to 5%,
Si: 0.1 to 1%,
and a balance of Fe and inevitable impurities, in which is uniformly distributed a hard dispersion phase of a Moxe2x80x94Fexe2x80x94Co alloy, having a composition comprising:
Fe: 20 to 30%,
Co: 13 to 22%,
Cr: 1 to 5%,
Si: 0.3 to 3%,
and a balance of Mo and inevitable impurities, and having a 2 phase mixed system of an Fexe2x80x94Co alloy phase and a Moxe2x80x94Co alloy phase, and
(c) infiltrating the Fe-based sintered alloy substrate with copper or a copper alloy.
As follows is a description of the reasons for restricting the compositions, average particle sizes, and mix proportions of the raw material powders, as well as the composition and porosity of the Fe-based sintered alloy substrate, to the values described above, for the process for producing a valve seat according to the present invention.
(A) Compositions of the Raw Material Powder for Forming the Matrix and the Fe-based Sintered Alloy Substrate:
(a) C
The C component in the substrate matrix following sintering is of the same content level as that of the raw material powder, and is dissolved in the matrix in a solid state, thereby strengthening the matrix, as well as forming carbides that are dispersed throughout the matrix thereby improving the wear resistance of the matrix. If a C component is also introduced into the hard dispersion phase, then it performs the function of improving the wear resistance of the hard dispersion phase. If the C content is less than 0.5%, then the actions described above do not provide the desired levels of improvement, whereas in contrast if the content exceeds 1.5%, counterpart attack increases rapidly. Accordingly, the C content was set within a range from 0.5 to 1.5%.
(b) Ni
Like the C component, the Ni component also remains in the substrate matrix, without diffusing and migrating into the hard dispersion phase, and is dissolved in the matrix in a solid state, thereby strengthening the matrix. If the Ni content is less than 0.1%, then the actions described above do not provide the desired effects, whereas in contrast if the content exceeds 3%, the strength deteriorates. Accordingly, the Ni content was set within a range from 0.1 to 3%.
(c) Mo
Like the C component and the Ni component, the Mo also remains in the substrate matrix during sintering, without diffusing and migrating into the hard dispersion phase, and is dissolved in the matrix in a solid state, while forming carbides that are dispersed throughout the matrix, thereby improving the strength and the wear resistance of the matrix. If the Mo content is less than 0.5%, then the actions described above do not provide the desired levels of improvement, whereas in contrast if the content exceeds 3%, the strength of the matrix deteriorates. Accordingly, the Mo content was set within a range from 0.5 to 3%.
(d) Co
The Co component of 3 to 8% incorporated within the raw material powder for forming the matrix combines with the large quantity of Co that diffuses and migrates from the hard dispersion phase during sintering, generating a Co content of 13 to 22% in the substrate matrix following sintering, thereby improving the high temperature corrosion resistance within the combustion gas atmosphere, whereas the diffusion and migration phenomenon described above improves the adhesion of the substrate matrix to the hard dispersion phase, thereby contributing to an improvement in wear resistance under conditions of high surface pressure application. If the Co content in the raw material powder for forming the matrix is less than 3%, then ensuring a Co content of at least 13% in the substrate matrix following sintering is extremely difficult, and the actions described above do not provide the desired effects, whereas if the Co content in the raw material powder for forming the matrix exceeds 8%, then the Co content in the substrate matrix following sintering can exceed 22% and become overly high, causing a deterioration in the wear resistance of the valve seat itself. Accordingly, the Co content of the raw material powder for forming the matrix was set within a range from 3 to 8%, and the Co content of the substrate matrix following sintering was set within a range from 13 to 22%.
(e) Cr
The Cr component of the raw material powder for forming the matrix is from 0.2 to 3%, whereas the substrate matrix following sintering incorporates from 1 to 5% due to diffusion and migration. If the Cr content in the raw material powder for forming the matrix is less than 0.2%, then the Cr content of the substrate matrix following sintering is less than 1%, and the solid solution strengthening of the matrix and the improvement in wear resistance arising from carbide formation are inadequate, whereas if the Cr content in the raw material powder for forming the matrix exceeds 3%, then the Cr content of the substrate matrix following sintering exceeds 5% and becomes overly high, causing a rapid increase in counterpart attack during application under conditions of high surface pressure application. Accordingly, the Cr content of the raw material powder for forming the matrix was set within a range from 0.2 to 3%, and the Cr content of the substrate matrix following sintering was set within a range from 1 to 5%.
(f) Si
The Si component incorporated within the substrate matrix is a component that has diffused and migrated from the hard dispersion phase during sintering, and as a result of the diffusion and migration of this Si component into the matrix substrate, the diffusion and migration of the Co component from the hard dispersion phase accelerates, and as a result, the adhesion of the hard dispersion phase to the substrate matrix improves markedly. If the Si content in the substrate matrix is less than 0.1%, then adequate diffusion and migration of the Co component into the substrate matrix may not be achievable, although the level of this migration is also related to the Si content within the raw material powder for forming the hard dispersion phase. In contrast, if the Si content in the substrate matrix exceeds 1%, then the strength of the matrix deteriorates. Accordingly, the Si content was set within a range from 0.1 to 1%.
(B) Compositions of the Raw Material Powder for Forming the Hard Dispersion Phase and the Substrate Hard Dispersion Phase
(a) Mo
The Mo component of the raw material powder for forming the hard dispersion phase forms a hard Moxe2x80x94Co alloy phase that represents one component phase of the two phase mixture of the substrate hard dispersion phase formed following sintering, and has a function of improving the wear resistance. If the content of the Mo component is less than 20% then the proportion of the Fexe2x80x94Co alloy phase that represents the other component phase becomes overly large, and the desired level of superior wear resistance cannot be ensured, whereas in contrast, if the Mo content exceeds 32%, then the sintering properties deteriorate, and achieving the desired strength for the valve seat becomes impossible. Accordingly, the Mo content of the raw material powder for forming the hard dispersion phase was set within a range from 20 to 32%.
(b) Cr
The Cr content of the raw material powder for forming the hard dispersion phase is from 5 to 10%, and a portion of this Cr component diffuses and migrates into the substrate matrix during sintering, producing a Cr content of 1 to 5% in the substrate matrix. If the Cr content in the raw material powder for forming the hard dispersion phase is less than 5%, then a Cr content of at least 1% in the substrate matrix following sintering cannot be achieved, and in such a case, as described above, the solid solution strengthening of the matrix and the improvement in wear resistance arising from carbide formation are inadequate. In contrast, if the Cr content in the raw material powder for forming the hard dispersion phase exceeds 10%, then the Cr content in the substrate matrix exceeds 5% and becomes overly high, causing a rapid increase in counterpart attack during application under conditions of high surface pressure application. Accordingly, the Cr content of the raw material powder for forming the hard dispersion phase was set within a range from 5 to 10%, and the Cr content of the substrate matrix following sintering was set within a range from 1 to 5%.
(c) Fe
The Fe component in the substrate hard dispersion phase is formed by diffusion and migration from the raw material powder for forming the matrix during sintering, and forms a very tough Fexe2x80x94Co alloy phase that represents one component phase of the two phase mixture of the substrate hard dispersion phase, and this Fexe2x80x94Co phase moderates the counterpart attack caused by the hard Moxe2x80x94Co alloy phase under conditions of high surface pressure application. If the Fe content of the substrate hard dispersion phase is less than 20%, then the proportion of the Moxe2x80x94Co alloy phase becomes overly large, and the desired level of counterpart attack moderation cannot be ensured, whereas in contrast, if the Fe content exceeds 30%, then the hardness of the substrate hard dispersion phase decreases, causing a deterioration in the wear resistance of the valve seat. Accordingly, the Fe content of the substrate hard dispersion phase was set within a range from 20 to 30%.
(d) Co
The Co component within the raw material powder for forming the hard dispersion phase forms the hard Moxe2x80x94Co alloy phase and the very tough Fexe2x80x94Co alloy phase, which represent the two component phases of the two phase mixture of the substrate hard dispersion phase formed following sintering, and this Co component improves the wear resistance while exhibiting a moderating effect on the counterpart attack properties. If the Co content of the substrate hard dispersion phase following sintering is less than 13%, then the strength of the Moxe2x80x94Fexe2x80x94Co alloy phase that comprises the 2 phase mixed system of the Moxe2x80x94Co alloy phase and the Fexe2x80x94Co alloy phase deteriorates, and the desired level of superior wear resistance for the valve seat cannot be ensured. In contrast, if the Co content exceeds 22%, then the hardness of the matrix hard dispersion phase itself deteriorates, and this also means that the desired level of superior wear resistance for the valve seat cannot be ensured. Accordingly, the Co content of the substrate hard dispersion phase was set within a range from 13 to 22%.
(e) Si
As described above, the Si component within the raw material powder for forming the hard dispersion phase undergoes diffusion and migration itself, and also promotes the diffusion and migration of the Co and Cr components in the raw material powder into the substrate matrix during sintering, thereby markedly improving the adhesion of the hard dispersion phase to the substrate matrix. If the Si content is less than 0.5%, then the diffusion and migration of the Co and Cr components into the substrate matrix is inadequate, making it impossible to ensure an excellent level of adhesion between the hard dispersion phase and the matrix. In contrast if the Si content exceeds 4%, then the Si component incorporated within the substrate matrix exceeds 1%, causing a deterioration in the strength of the substrate matrix. Accordingly, the Si content of the raw material powder for forming the hard dispersion phase was set within a range from 0.5 to 4% (resulting in a Si content of the substrate hard dispersion phase following sintering within a range from 0.3 to 3%).
(C) Raw Material Powders
(a) Average Particle Sizes
The average particle sizes for both the raw material powder for forming the matrix and the raw material powder for forming the hard dispersion phase are within a range from 20 to 50 xcexcm. If the average particle size is either less than 20 xcexcm, or greater than 50 xcexcm, then the diffusion and migration of the Co component from the raw material powder for forming the hard dispersion phase into the substrate matrix become difficult, meaning the mutual diffusion and migration of the Fe component from the raw material powder for forming the matrix into the hard dispersion phase is also unsatisfactory. As a result, the adhesion of the hard dispersion phase to the substrate matrix following sintering is unsatisfactory, and wear progresses at a markedly quicker rate under conditions of high surface pressure application. Accordingly, the average particle size of each of the raw material powders was set within a range from 20 to 50 xcexcm.
(b) Mix Proportion of the Raw Material Powder for forming the Hard Dispersion Phase
If the mix proportion of the raw material powder for forming the hard dispersion phase is less than 25% by weight, then the desired level of wear resistance cannot be ensured, whereas if the mix proportion exceeds 35% by weight, then not only does the counterpart attack increase rapidly, but the strength also decreases. Accordingly, this mix proportion for the raw material powder for forming the hard dispersion phase was set within a range from 25 to 35% by weight relative to the combined quantity with the raw material powder for forming the matrix.
(D) Porosity of the Fe-based Sintered Alloy Substrate
If this porosity is less than 5%, infiltration of copper and copper alloys is non-uniform, and the effect of this infiltration is not adequately displayed, whereas if the porosity exceeds 15%, then reductions in the strength and wear resistance become unavoidable. Accordingly, the porosity was set within a range from 5 to 15%.
(E) The Aforementioned Vacuum Refers to an Atmosphere of no More than 100 Pa. The temperature range for the sintering is preferably from 1100 to 1250xc2x0 C., and the time for which the sintering temperature is maintained is preferably from 0.5 to 2 hours.